1. Field of Invention
The present invention generally relates to optical fiber communication networks, and more particularly to a splitting system for a multi-channel optical fiber communication network for multiplexing and/or de-multiplexing an optical signal with a plurality of individual channels and various wavelengths.
2. Description of the Related Art
The telecommunications network serving the United States and the rest of the world is presently evolving from analog to digital transmission with ever increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
The T-1 standards committee ANSI has provided a draft document, xe2x80x9cANSI T1.105-1988xe2x80x9d, dated Mar. 10, 1988, which sets forth specifications for rate and format of signals which are to be used in optical interfaces. The provided specifications detail the Synchronous Optical Network (SONET) standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS0 and DS1 signals into a common medium. In essence, SONET established a uniform standardization transmission and signaling scheme, which provided a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished.
Currently this expansion of bandwidth is being accomplished by what is known as xe2x80x9cwavelength division multiplexingxe2x80x9d (WDM), in which separate subscriber/data sessions may be handled concurrently on a single optic fiber by means of modulation of each of those subscriber datastreams on different portions of the light spectrum. Therefore, WDM is the optical equivalent of frequency division multiplexing (FDM). Current implementations of WDM involve as many as 128 semiconductor lasers each lasing at a specific center frequency within the range of 1525-1575 nm. A wavelength division multiplexer (WDM) is usually a passive optical network or device. The WDM can be used to divide wavelengths (or channels) from a multi-channel optical signal or to combine various wavelengths (or channels) on respective optical paths into one multi-channel optical signal on one optical path. Each subscriber datastream is optically modulated onto the output beam of a corresponding semiconductor laser. The modulated information from each of the semiconductor lasers is combined onto a single optic fiber for transmission.
There are three classes of WDM""s: coarse, intermediate, and dense. Coarse WDM""s are configured for dividing and combining two wavelengths (or channels) that are spaced relatively far apart, e.g., 1310/1550 nanometers. The WDM is used to separate wavelength bands (with 100 nm, i.e., 13 terahertz bandwidth) centered around 1310 nm and 1550 nm. Intermediate WDM""s are configured for dividing and combining two or three wavelengths (or channels) that are spaced closer than those of the coarse WDM""s, e.g., a 1540/1560 nm WDM used to put to channels approximately 20 nm, i.e. 2.5 terahertz, apart in the 1550 nm wavelength bands. Currently, a third category, dense WDM""s (also referred to as DWDM""s) are configured for dividing and combining 4, 8, 16, 32, 64, 128 or more wavelengths (or channels) that are very closely spaced. The spacing between channels is constantly being reduced as the resolution and signal separation capabilities of multiplexers and de-multiplexers are improved. Current International Telecommunications Union (ITU) specifications call for channel separations of approximately 0.4 nm, i.e., 50 GigaHertz. At this separation, as many as 128 channels may be carried by a single fiber in a bandwidth range within the same capacity of an erbium doped fiber amplifier (EDFA).
Because of the close spacing between the channels in a DWDM, it is desirable to design a DWDM with flat pass bands in order to increase the error tolerance. This is primarily because the center wavelength of a pass band slips with temperature, usually on the order of about 0.011 nm, i.e., 1.4 GigaHertz per degree centigrade. Further, the cascading of the DWDM stages causes the pass bands to become narrower and each DWDM down the chain. Therefore, the larger the pass bands the greater the shift tolerance of the channel.
Further, it is desirable to design a DWDM with low loss to leave more room for loss in other components in network. For example, if losses are reduced, the distance the fiber reaches, i.e. the span, can be longer. As another example, if the power of the transmitting lasers is allowed to be reduced, then inexpensive lasers can be used.
Various devices, such as multi-stage band and comb splitters, have been proposed to fill these new demanding requirements and none are fully satisfactory.
In a multi-stage band splitter, the first stage makes a coarse split of two wavelength ranges, and subsequent stages make finer and finer splits of sub-bands within each of the wavelength ranges. In this scheme, the WDM""s of the subsequent stages are largest in quantity and the most expensive to fabricate because they have the smallest channel spacing.
In a multi-stage comb splitter, the first de-multiplexing stage filters out two interlaced periodic sets of relatively narrow band passes and the subsequent stages employ wider band pass periodic filters until the individual channels are de-multiplexed.
In either case, noise and inter-channel interference are limiting factors in the handling of increasingly narrow band pass requirements. Multi-layer thin-film filters can be used to construct optical filters in bulk optics, but they are undesirable because of an increase in the number of layers, precision of manufacture and expense associated with increasingly narrow band pass requirements. Additionally, thin-film optical filters cannot be readily integrated because of difficulties in coupling to and from fibers. Mach-Zehnder interferometers have been widely employed, but they have a sinusoidal response, giving rise to strongly wavelength dependent transmission and a narrow rejection band. Other designs have encountered a variety of practical problems. Accordingly, there is a need for the new type of optical multiplexer/de-multiplexer which can be easily fabricated, with narrower band pass capabilities.
A method and apparatus for multiplexing/de-multiplexing optical signals is disclosed. The method and apparatus are applicable to a range of optical multiplexing techniques including, but not limited to: wavelength division multiplexing (WDM), dense wavelength division multiplexing (DWDM) and frequency division multiple accessing (FDMA). The disclosed devices do not require active components. The disclosed devices may be implemented with fiber or fiberless optical communications systems including telecommunication systems. The devices may be used on their own or as part of a larger system such as a multi-stage mux/demux, an optical switch or router. The devices exhibit a small form. They may be precisely tuned to a specific wavelength grid. Additionally, the devices are passively thermally stabilized with the result that their tuning is substantially invariant across a wide temperature range.
In an embodiment of the invention, an optical device for operating on optical signals between a first port communicating odd channels and even channels and a second port communicating the odd channels together with a third port communicating the even channels is disclosed. Adjacent orders of the odd and even channels are evenly spaced apart and each centered on a corresponding gridline of a selected wavelength grid. The optical device includes a linear polarizer, at least one wave plate, and a beam displacer/combiner. The linear polarizer couples to the first port for linearly polarizing optical signals. The at least one wave plate include a first and a second I/O interface. The at least one wave plate optically couples at a first I/O interface to the linear polarizer. The at least one wave plate rotates both odd and even channel components of an optical beam between a linear and an orthogonal relationship depending on the propagation direction. The at least one wave plate includes a primary and a compensating birefringent element. The primary birefringent element with a selected first length and first indices of refraction. The compensating birefringent element with a selected second length and a second indices of refraction distinct from the first indices of refraction. The first length, the second length, the first indices of refraction and the second indices of refraction together determinative both of a free spectral range for the at least one wave plate corresponding to a spacing between adjacent gridlines of the selected wavelength grid, and determinative as well as of a combined optical pathlength difference for the wave plate substantially invariant with respect to temperature. The beam displacer/combiner optically couples the second I/O interface of the plurality of wave plates with both the second port and the third port. The beam displacer/combiner displaces and combines orthogonally polarized odd and even channel components of an optical beam depending on propagation direction.
In an alternate embodiment of the invention, a method for splitting/combining odd and even channels of an optical beam between a first port communicating odd channels and even channels and a second port communicating the odd channels together with a third port communicating the even channels is disclosed. Adjacent orders of the odd and even channels are evenly spaced apart and each centered on a corresponding gridline of a selected wavelength grid. The method for splitting/combining comprises;
linearly polarizing the optical beam at the first port;
relatively retarding an e-ray and an o-ray component of the optical beam by a primary amount and a compensating amount and a sum of the primary amount and the compensating amount substantially invariant with respect to temperature, and the sum corresponding to an integer multiple of 2xcfx80 of a one of the odd channels and the even channels and to odd integer multiples of xcfx80 for an other of the odd channels and the even channels, both at a free spectral range corresponding to the spacing between gridlines of the selected wavelength grid, to rotate odd and even channel components of the optical beam between a linear and an orthogonal relationship depending on the propagation direction; and
displacing and combining orthogonally polarized odd and even channel components of an optical beam depending on a propagation direction.
In an embodiment of the invention, an optical device for operating on optical signals between a first port communicating odd channels and even channels and a second port communicating the odd channels together with a third port communicating the even channels is disclosed. Adjacent orders of the odd and even channels are evenly spaced apart and each centered on a corresponding gridline of a selected wavelength grid. The optical device includes at least one wave plate. The at least one wave plate include a first and a second I/O interface. The at least one wave plate rotates both odd and even channel components of an optical beam between a linear and an orthogonal relationship depending on the propagation direction. The at least one wave plate includes a primary and a compensating birefringent element. The primary birefringent element with a selected first length and first indices of refraction. The compensating birefringent element with a selected second length and a second indices of refraction distinct from the first indices of refraction. The first length, the second length, the first indices of refraction and the second indices of refraction together determinative both of a free spectral range for the at least one wave plate corresponding to a spacing between adjacent gridlines of the selected wavelength grid, and determinative as well as of a combined optical pathlength difference for the wave plate substantially invariant with respect to temperature.
In an embodiment of the invention, an optical device for operating on optical signals between a first port communicating odd channels and even channels and a second port communicating the odd channels together with a third port communicating the even channels is disclosed. Adjacent orders of the odd and even channels are evenly spaced apart and each centered on a corresponding gridline of a selected wavelength grid. The optical device includes a linear polarizer and a plurality of crystals. The linear polarizer couples to the first port for linearly polarizing the optical signals. The plurality of crystals oriented to intersect a path of an optical beam. The plurality of crystals include a first and a second I/O interface. The plurality of crystals couple at a first I/O interface to the linear polarizer. The plurality of crystals rotate both odd and even channel components of an optical beam between a linear and an orthogonal relationship depending on the propagation direction and substantially independent of a polarization orientation of the linearly polarized optical signals at the first I/O interface.